The objectives are to analyze the role of rDNA in stimulation of asynoptonemal X:Y pairing and to explore the maintenance of rDNA sequence homogeneity through non-allelic recombination in Drosophila. The experiments are motivated in part by the observation, described in Preliminary Studies, that a cloned transformed rDNA cistron stimulates X:Y pairing when located at a euchromatic site on a pairing deficient X chromosome but not when located on an autosome. Cloned rDNA cistrons transformed into the germ line via P-element vectors will be a) remobilized to obtain new X-linked insertions, and b) destabilized to obtain overlapping internal deletions, using a high efficiency hybrid dysgenesis method. The resulting variants will be mapped and analyzed at the molecular level and used to: 1) assess the relative importance of DNA homology versus gene expression in promotion of X:Y pairing by comparing rDNA sequence requirements for pairing stimulation, transcription, nucleolus formation, and ribosome assembly. 2) test for position and copy number effects on rDNA stimulation of X:Y pairing and rDNA expression. 3) determine the rate and mechanism of rDNA recombination by screening for exchanges and conversions between allelic and non- allelic euchromatic rDNA cistrons. 4) test for magnification (heritable amplification triggered by partial rDNA deficiency) of isolated euchromatic cistrons 5) assess the relative importance of homology versus expression in recombination and magnification by identifying sequence requirements via deletion mapping. In addition, clones of ribosomal insertion sequences will be tested by P-element transformation for effects on X:Y pairing. This research has possible significance for human health. Participation of nucleoli in meiotic chromosome pairing has been linked to non-homologous segregation of nucleolus organizer- containing chromosomes, leading to nondisjunction and Down's syndrome. In addition, non-allelic homologous recombination between dispersed repeated genes is a likely cause of chromosome rearrangements responsible for birth defects and cancer.